The present invention generally relates to the processing of gold ore, and more particularly to a method in which gold ore having copper therein is treated to effectively separate gold from copper. As a result, a purified elemental gold product can be produced from impure ore materials in a highly effective and economical manner which avoids excessive reagent (e.g. cyanide) consumption.
In order to recover elemental gold (Au) from gold-containing ore, traditional methods involve treating the ore with one or more aqueous (e.g. water-containing) cyanide-containing leaching solutions, with this term encompassing a wide variety of different dissolved cyanide compounds including sodium cyanide (NaCN), potassium cyanide (KCN), and calcium cyanide Ca(CN).sub.2 !. Other cyanide-containing materials (e.g. cyanide compounds) which may be used for this purpose include but are not limited to gaseous hydrogen cyanide (HCN.sub.(g)), ammonium cyanide (NH.sub.4 CN), organic alpha-hydroxy cyanides (e.g. lactonitrile), and/or thiocyanates (e.g. NaSCN, KSCN, or Ca(SCN).sub.2.) As a result, a "gold-cyanide complex" is produced during contact between the ore and the leaching solution with this term being defined to involve a chemical complex containing one or more gold ions stoichiometrically combined with at least one or more cyanide ions (CN).sup.- !. This complex will typically consist of Au(CN).sub.2.sup.-1 (also known as an "aurocyanide ion") which is associated with one or more counter-ions including, for example, Na.sup.+ when NaCN is employed in the leaching solution, K.sup.+ when KCN is used, and Ca.sup.+2 when Ca(CN).sub.2 is involved. The Au(CN).sub.2.sup.-1 complex has a high level of stability with a K.sub.f of about 2.times.10.sup.38. A typical reaction sequence in which a gold-cyanide complex is produced using a selected cyanide ion-containing leaching solution is as follows: EQU 4Au.sub.(s) +8(CN).sup.-.sub.(aq) +O.sub.2(aq) +2H.sub.2 O.sub.(1) .fwdarw.4Au(CN).sub.2.sup.-.sub.(aq) +4OH.sup.-.sub.(aq) ( 1)
This reaction is further described in Brown, T. L., et al., Chemistry, The Central Science, Prentice Hall, New Jersey, 4th ed., p. 815 (1988). It is important to emphasize that the foregoing reaction will occur when a wide variety of different cyanide-containing leaching solutions are employed, with the present invention not being restricted to the use of any particular materials for this purpose. In addition, if needed and desired as determined by preliminary testing and analysis, the cyanide-containing leaching solution is maintained at an alkaline pH (optimum=about 9-11) using lime (CaO) to maintain a high rate of gold cyanidation.
A variety of different physical methods may be employed to place the gold ore in contact with the selected cyanide-containing leaching solution. Two methods of primary commercial interest involve procedures known as (1) "heap leaching"; and (2) "vat leaching". In both of these processes, crushed gold ore is combined with the selected cyanide-containing leaching solution which is allowed to pass through the ore so that gold extraction/complex formation can take place. In heap leaching systems, individual rock-like portions of gold-containing ore are initially provided, with each portion being about 1-4 inches in diameter. The rock-like portions of ore are then placed in a pile which is typically positioned on a pad made of rubber or the like. In a representative and non-limiting embodiment, each pile is normally about 30-50 ft. tall and occupies about 1.times.10.sup.7 to 3.times.10.sup.7 ft.sup.3 of space, although these values may be varied as needed in accordance with the size and capacity of the processing facility under consideration. The selected cyanide-containing leaching solution is then applied to the top of the ore pile and allowed to travel (e.g. percolate) downwardly therethrough. During this procedure, the leaching solution passes into the interior regions of the individual ore portions (rocks) which have a porous character. As a result, the liquid materials leaving the ore pile at the bottom thereof consist of an aqueous solution containing a gold-cyanide complex (described above). Further processing of the gold-cyanide complex to obtain elemental gold therefrom will be discussed in substantial detail below.
The foregoing procedure (e.g. placing gold ore in contact with a cyanide-containing leaching solution) may likewise be undertaken in a large containers or "vats" which are entirely or partially closed. These vats are typically constructed from stainless steel or lined carbon steel and have a representative capacity of about 400-2500 ft.sup.3 in a non-limiting and preferred embodiment. Likewise, instead of using "rock"-type portions of ore as discussed above, powdered ore may also be treated in a vat or heap system as discussed in U.S. Pat. No. 5,264,192. In this embodiment, mined ore in the form of large rocks is crushed using conventional mechanical systems (e.g. jaw-crushers, roll-crushers, and/or attrition mills which are known in the art and of standard design). As a result, a powered ore product is generated which has an average particle size of about 200 U.S. standard mesh or less. The powdered ore is thereafter treated with a selected cyanide-containing leaching solution as previously noted.
Heap or vat leaching processes which incorporate cyanide extraction technology are currently in widespread use throughout the United States and in other countries. For example, in 1989, the United States had about eighty heap or vat leaching operations, with most of them being located in Nevada. Other large leaching operations currently exist in Peru, Ecuador, Chile, South Africa, Indonesia, Canada, and elsewhere.
The present invention as described in considerable detail below shall not be restricted to any specific leaching procedures (e.g. heap leaching, vat leaching, and the like), any particular cyanide-containing leaching solutions, or any physical parameters (e.g. size characteristics) associated with the gold ore being treated. The invention is applicable to any leaching method which places a cyanide-containing leaching solution in direct physical contact with gold ore to yield an aqueous product containing a gold-cyanide complex. Further general information regarding the gold leaching processes described above and operational parameters associated with these procedures (including specific examples) are presented in U.S. Pat. No. 5,264,192; Thomas, R. (ed.), E/MJ Operating Handbook of Mineral Processing, McGraw-Hill, Inc., pp. 22-23 (1977); Clennell, J., The Cyanide Handbook, McGraw-Hill, Inc. pp. 102-132 (1915); and Bernard, G. M. "Andacollo Gold Production--Ahead of Schedule and Under Budget", Mining Engineering, pp. 42-47 (August 1996) which are all incorporated herein by reference.
Once the desired gold-cyanide complex is generated using the processes discussed above, it must thereafter be treated to recover elemental gold therefrom. This can be done immediately or after the passage of a predetermined amount of time. While a number of different procedures may be employed for this purpose, two primary methods exist which are currently in widespread use. These methods are known as (1) the "Merrill-Crowe Process"; and (2) the "Activated Carbon Process". The Merrill-Crowe Process is described in numerous references including Arbiter, H., et al., Gold--Advances in Precious Metals Recovery, Gordon and Breach Science Publishers, New York, pp. 146-153 (1990); and Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, Society of Mining Engineers, Inc., Littleton, Colo., pp. 126-127 and 149-150 (1988) which are also incorporated herein by reference.
The Merrill-Crowe Process (which was initially developed in approximately 1897) involves a procedure in which the "pregnant" leaching solution (which contains the desired gold-cyanide complex therein) undergoes a reaction conventionally known as "zinc cementation/precipitation". Specifically, the leaching solution containing the gold-cyanide complex is combined with elemental zinc (Zn) in accordance with the following reaction: EQU 2Au(CN).sub.2.sup.-1.sub.(aq) +Zn.sub.(s) .fwdarw.2Au.sub.(s) +Zn(CN).sub.4.sup.-2.sub.(aq) ( 2)
Various lead salts (e.g. lead acetate and/or lead nitrate) may also be added to the foregoing reaction process as needed in accordance with preliminary pilot tests in order to increase the reaction kinetics of the gold precipitation process. Implementation of this technique generates solid elemental gold (Au) which resides within a gold-zinc solid sludge-type reaction product. This material is ultimately filtered and removed from the residual liquid fraction (which consists primarily of free cyanide ions (CN).sup.- } and a dissolved Zn(CN).sub.4.sup.-2.sub.(aq) complex.) The zinc-gold solid sludge is thereafter processed to isolate and remove elemental gold therefrom. A number of different methods may be employed for this purpose. For example, after being washed with water to remove residual free cyanide ions and Zn(CN).sub.4.sup.-2.sub.(aq) complex, the reaction product may then be combined with sulfuric acid (H.sub.2 SO.sub.4) in the presence of air in order to dissolve excess (unreacted) elemental zinc and other metals including copper and cadmium as discussed in Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, p. 150. The remaining solid materials are thereafter washed with water again and dried. If it is determined by preliminary experimental testing that the solid product contains substantial quantities of mercury (Hg), then the product may be further processed in a conventional mercury retort at 400.degree. C. to release residual mercury into a condenser assembly which is optimally positioned under water to avoid the release of vaporized mercury into the atmosphere. In the alternative, as discussed in Brown, T. L., et al., Chemistry, The Central Science, supra, p. 815, the sludge-like reaction product may be heated in air to form zinc oxide (ZnO) from residual elemental zinc which is thereafter sublimed away.
The elemental gold-containing solid product which results from the procedures listed above may then be smelted in combination with a selected flux composition which is designed to oxidize elemental zinc (as well as other residual non-gold metals) and thereby assist in the removal of metal oxides. Representative flux materials suitable for this purpose include but are not limited to "borax" (e.g. Na.sub.4 B.sub.4 O.sub.7.10H.sub.2 O) and silica (e.g. SiO.sub.2) in combination. The specific flux materials and combinations thereof, as well as the amounts of these materials to be used in the smelting process will be determined in accordance with preliminary pilot studies on the gold-containing solid product being processed. Likewise, specific information on the use of flux materials in general is again presented in Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, p. 150. Addition of the flux materials as discussed above generates a borosilicate glass "slag", with this term being defined to involve a relatively inert reaction product created when flux materials are combined with impurities in a metal refining system. It should also be noted that, if needed as determined by preliminary pilot testing, feldspar (which comprises a silicate of aluminum and possibly other metals) may be added at approximately a 3% by weight level as a viscosity modifier.
After the steps listed above, smelting of the reaction product is initiated which takes place in a conventional furnace (e.g. a gas-fired or induction-type furnace system which is known in the art) at a temperature of approximately 1150.degree. C. Finally, after removing the residual "slag" which gravimetrically separates and collects in the furnace, the elemental gold (characterized as "dore") is withdrawn from the furnace, thereby completing the production process. Again, this basic refining procedure is conventional in nature and discussed in substantial detail in the foregoing references including Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra.
The Activated Carbon Process employs a different approach. Specifically, the aqueous leaching product/solution having the gold-cyanide complex dissolved therein is placed in contact with activated carbon which is typically positioned in large column-like structures. The term "activated carbon" as used herein involves carbon materials having an amorphous character, a large surface area, and a considerable number of pores or "activation sites". Activated carbon which is suitable for use in this process may be obtained from the charring of coconut shells or peach pits at approximately 700-800.degree. C. and will typically have the following optimum parameters: (1) surface area=1050-1150 m.sup.2 /gm; (2) apparent density=0.48 g/cc; (3) particle density=0.85 g/cc; (4) voids in densely packed column=40%; and (5) representative particle sizes=minus 6-plus 16 mesh or minus 12-plus 30 mesh. However, the present invention and activated carbon adsorption processes in general shall not be restricted to these particular parameters which are provided for example purposes only.
Once the aqueous leaching solution containing the gold-cyanide complex therein comes in contact with the activated carbon, an adsorption process occurs which is not yet entirely understood. Specifically, the gold-cyanide complex in solution (which is defined herein to encompass aurocyanide ions, namely, Au(CN).sub.2.sup.-1) is adsorbed onto the surface of the activated carbon in accordance with a number of theoretical mechanisms including the possible presence of multiple "surface oxide sites" which enable adsorption to take place. This mechanism, as well as additional information regarding the Activated Carbon Process, is presented in Arbiter, H., Gold--Advances in Precious Metals Recovery, supra, pp. 153-164; and Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, pp. 128-129; 138-149; and 151 which are again incorporated herein by reference. Generally, the activated carbon supplies which are employed in this method are operated in a "fluidized bed" mode which may be achieved through the use of a liquid flow rate of about 25 gpm/ft.sup.2 of cross-sectional area associated with the carbon-containing column (or other support structure) when minus 6-plus 16 mesh particles are employed. When minus 12-plus 30 mesh carbon is used, a flow rate of about 15 gpm/ft.sup.2 is preferred. Both of these parameters will typically result in a bed expansion of about 60%.
Regardless of which mechanism ultimately results in adsorption of the gold-cyanide complex (e.g. aurocyanide ions) on the activated carbon, the following approach may be used to effectively removes the gold-cyanide complex from the aqueous leaching product. After adsorption, the gold-containing carbon product is filtered to remove residual "barren" liquid, followed by "desorption" or removal of the gold-cyanide complex from the "loaded" activated carbon (e.g. the gold-containing carbon product.) This is accomplished by using a selected eluant solution which is placed in direct physical contact with (e.g. passed through) the carbon. A representative eluant solution that is suitable for this purpose includes but is not limited to a solution of NaOH--NaCN (e.g. optimally about 0.5-1.0% by weight NaOH and about 0.1-0.3% by weight NaCN containing approximately 20% ethyl alcohol) as specifically mentioned in Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, p. 139. This solution is likewise heated in a preferred embodiment to a temperature of about 77-120.degree. C. It is theorized that cyanide ions (CN).sup.- ! in the eluant solution effectively replace/exchange the adsorbed aurocyanide ions (gold-cyanide complex) which are released into the eluant solution. The resulting gold-containing eluant product (which contains the desired gold species aurocyanide ions/gold cyanide-complex!) is then further processed to recover elemental gold therefrom. At this point, it is important to emphasize that the overall gold concentration in the gold-containing eluant product is substantially greater than the gold concentration in the original leaching solution, thereby demonstrating the effectiveness of this procedure in producing a concentrated gold product. For example, as noted in Arbiter, H., Gold--Advances in Precious Metals Recovery, supra, p. 144, a representative leaching solution (after gold extraction) will have an overall gold concentration of about 1-10 ppm while an exemplary gold-containing eluant product as discussed above will comprise about 100-2000 ppm of gold therein.
At this point, the gold-containing eluant product is treated to recover elemental gold therefrom. This may again be accomplished in many ways (including zinc precipitation in accordance with the Merrill-Crowe Process as outlined above), although conventional electrowinning methods are preferred as again discussed in Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, pp. 143-148 and 151. While electrowinning is a known procedure that has been employed in the mining industry for decades, the specific details of this process will now be summarized. First, an electrowinning "cell" is provided which includes one or more cathodes and anodes therein. Both of these elements are in fluid communication with the gold-containing eluant solution which is supplied to the cell housing having the cathodes and anodes therein. A direct current power supply is then operatively connected to the cathodes and anodes in each cell which causes the desired metal in the solution (e.g. elemental gold in the gold-containing eluant product) to be directly deposited onto the cathodes. This process shall not be restricted to any particular materials which may be used in connection with the cathodes and anodes, with a wide variety of conventional compositions being suitable for this purpose. In a representative and non-limiting embodiment, cathodes manufactured from steel wool (e.g. positioned in a plastic frame or wrapped around a stainless steel spool) and anodes produced from stainless steel, carbon, or titanium can be employed. Many different sizes, shapes, and overall design configurations may be selected in connection with the cathodes/anodes, with the claimed process (and the electrowinning procedure in general) not being restricted to any particular structures and physical parameters. Likewise, the power required for electrowinning will vary in accordance with many factors including the particular type of cell(s) under consideration, the gold concentration in the gold-containing eluant product, the construction materials associated with the cathodes/anodes, and the like. However, a representative system will involve the application of approximately 2.5 volts between the cathodes and anodes in an exemplary electrowinning cell.
Once the electrowinning process is completed, the elemental gold-containing cathodes are removed from the system and treated to recover elemental gold therefrom. The cathodes at this stage may contain up to about 50% or more gold thereon (e.g. up to about 100 oz. of elemental gold per lb. of cathode if steel wool is involved). To process the cathodes, they may initially be placed in contact with sulfuric acid (H.sub.2 SO.sub.4) in an optional pretreatment step which is designed to dissolve any residual non-gold metals including copper, iron, and the like. The need for a sulfuric acid pretreatment stage is typically determined in accordance with preliminary pilot studies on the electrowinning products (e.g. cathodes) under consideration. Likewise, if the cathodes contain substantial amounts of mercury (which will not usually be removed by sulfuric acid treatment), they may be subjected to conventional retort processes as discussed above. The cathodes are then smelted in combination with one or more selected flux compositions which are again designed to oxidize residual non-gold metals and thereby assist in the removal of metal oxides. Representative flux compounds suitable for this purpose include but are not limited to "borax" (e.g. Na.sub.4 B.sub.4 O.sub.7.10H.sub.2 O) and silica (e.g. SiO.sub.2) in combination. The specific flux materials and combinations thereof, as well as the amounts of these materials to be used in the smelting process will be determined in accordance with preliminary pilot studies on the gold-containing cathode materials under consideration. More detailed information on the use of flux materials for this purpose is again presented in Van Zyl, D. J. A., et al., Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, supra, pp. 150-151. Addition of the flux materials results in the production of a borosilicate glass "slag" with this term being defined above. It should also be noted that, if needed as determined by preliminary pilot testing, feldspar may be added at approximately a 3% by weight level as a viscosity modifier.
After the steps listed above, smelting of the cathodes is initiated which takes place in a conventional furnace (e.g. a gas-fired or induction-type furnace system that is known in the art) at a temperature of approximately 1150.degree. C. Finally, after removing the residual "slag" which gravimetrically separates and collects in the furnace, the elemental gold (e.g. characterized as "dore") is withdrawn from the furnace, thereby completing the production process. Again, this basic refining procedure is conventional in character and discussed in the references listed above.
Both the Merrill-Crowe Process and the Activated Carbon Process involve established procedures for collecting and isolating gold-containing species (e.g. aurocyanide ions) from cyanide-based leaching solutions so that elemental gold can be recovered. Further information regarding these procedures will be presented below in the Detailed Description of Preferred Embodiments section. It is likewise important to emphasize that the present invention shall not be restricted to any particular gold collection/isolation techniques. The claimed method is instead prospectively applicable to any technique for obtaining an elemental gold product from cyanide solutions used in heap leaching processes, vat leaching methods, and other cyanide-based extraction systems. For example, in addition to the Merrill-Crowe Process and the Activated Carbon Process (which are both preferred), other representative methods of a conventional nature which may be employed to collect and isolate gold-cyanide complexes (e.g. aurocyanide ions), following by additional purification to yield elemental gold include (1) solvent extraction procedures which use alkyl phosphorus esters, as well as primary, secondary, tertiary, and/or quaternary amines (alone or combined with phosphine oxides, sulfones, and/or sulfoxides) to extract gold-cyanide complex materials from leach solutions; and (2) ion exchange methods and compositions (e.g. resins) in which aurocyanide ions are extracted from cyanide-based leaching solutions, with representative elution materials suitable for use with these compositions including sodium hypochlorite, zinc cyanide, thiocyanate, a mixture of thiocyanate/dimethyl formamide ("DMF"), and the like. Exemplary ion exchange resins which may be employed for this purpose include those sold under the trademark DOWEX and others which are commercially available from the Dow Chemical Company of Midland, Mich. (USA). Both of these gold isolation methods (combined with conventional electrowinning and smelting processes as discussed above) represent alternative procedures which may be employed to isolate and collect elemental gold from cyanide-containing leaching solutions. These alternative techniques are discussed in Arbiter, H., Gold--Advances in Precious Metals Recovery, supra, pp. 164-185. In accordance with the information provided above, the present invention shall therefore not be restricted to any particular methods for isolating elemental gold from leaching solutions containing gold-cyanide complexes therein, with the versatility of the claimed process becoming readily apparent from the specific information provided below in the Detailed Description of Preferred Embodiments section.
Regardless of which methods are ultimately used to obtain elemental gold from gold-cyanide complexes, numerous technical and economic problems can result in various portions of the leaching system when gold ore is processed which contains substantial amounts of elemental copper. Copper-containing gold ore is obtainable from many countries throughout the world including Australia, Chile, Philippines, Saudi Arabia, Canada, Argentina, Indonesia, Peru, and Mexico. Significant problems will result when the copper-containing gold ore contains about 0.1-2.0% by weight elemental copper or more, although the claimed process shall not be limited to the treatment of ore containing any particular copper levels. In all cyanide-based leaching processes (including heap leaching and vat leaching systems), material costs represent a substantial portion of the overall operating expense in the processing of gold ore. These material costs are primarily associated with the cyanide-containing leaching solution as discussed above. The excessive consumption of cyanide materials during ore treatment will substantially reduce the operating efficiency of the entire gold production facility. It is therefore a goal of all cyanide based leaching operations to minimize the use of cyanide compositions (e.g. cyanide salts dissolved in aqueous solutions) and to avoid excessive losses of these materials.
However, when copper is present in the gold ore as indicated above, a chemical "side-reaction" occurs which results in excessive consumption and losses of the cyanide-containing leaching solution. Undesired and excessive consumption of cyanide ions (CN).sup.- ! which takes place when elemental copper is present in the gold ore involves the following chemical reaction: EQU 2Cu.sub.(s) +6(CN).sup.-.sub.(aq) .fwdarw.2Cu(CN).sub.3.sup.-2.sub.(aq)( 3)
This reaction consumes substantial amounts of free cyanide (CN).sup.- ! in order to produce a copper-cyanide complex, thereby increasing the overall cyanide requirements in the leaching process. The term "copper-cyanide complex" as used herein shall be defined to involve a chemical complex containing one or more copper ions stoichiometrically combined with at least one or more cyanide ions (CN).sup.- !. This complex will primarily consist of Cu(CN).sub.3.sup.-2 (also known as a "cuprocyanide ion") which is associated with one or more counter-ions including, for example, Na.sup.+ when NaCN is employed in producing the leaching solution, K.sup.+ when KCN is used, and Ca.sup.+2 when Ca(CN).sub.2 is involved. Much of the copper-cyanide complex (Cu(CN).sub.3.sup.-2) which is generated as a result of this reaction passes unaffected through the gold extraction and isolation processes outlined above, and ultimately resides in the "barren" cyanide-containing solution materials which remain after the gold-cyanide complex is removed. This barren solution is normally reused/recycled in treating incoming amounts of additional gold ore. However, when the barren solution contains the copper-cyanide complex therein, this material (Cu(CN).sub.3.sup.-2) decomposes via oxidation or other processes to Cu(CN).sub.2.sup.-1 during exposure to air, bacterial action, and/or sunlight in storage ponds, on heaps of gold ore, and the like. This copper-cyanide compound (Cu(CN).sub.2.sup.-1) represents a considerable problem in the recycled barren cyanide solution since Cu(CN).sub.2.sup.-1 is chemically incapable of extracting gold from gold ore to yield the desired gold-cyanide complex and recombines with fresh cyanide ions (CN).sup.- ! that are added during the reuse and recycling of the barren solution. The Cu(CN).sub.2.sup.-1 is then reconverted back into the copper-cyanide complex (Cu(CN).sub.3.sup.-2), thus consuming two moles of (CN).sup.-. As more copper leaches into the recirculating leaching solution (which occurs during reuse of this material and repeated passage thereof through incoming quantities of gold ore), increasingly large amounts of cyanide are irreversibly lost to this decomposition/(CN).sup.- consumption cycle. The presence of copper in the gold ore being treated therefore presents significant problems from a functional and economic standpoint.
In summary, the presence of elemental copper in the gold ore being treated ultimately increases cyanide consumption in the system by forming a copper-cyanide complex (Cu(CN).sub.3.sup.-2) which can decompose to yield Cu(CN).sub.2.sup.-1. The Cu(CN).sub.2.sup.-1 in the recycle stream, when "conditioned" by the addition of fresh (CN).sup.- is reconverted to Cu(CN).sub.3.sup.-2 resulting in the loss of two moles of (CN).sup.- to the inert Cu(CN).sub.3.sup.-2 compound. This loss (which involves excessive (CN).sup.- reagent consumption) significantly and adversely affects the cost efficiency of the entire gold processing operation.
In addition to excessive cyanide consumption, copper materials within the gold ore will also result in an increasingly impure elemental gold product. Additional and more costly refining procedures must therefore be employed to solve this problem. Likewise, if the Merrill-Crowe Process is used (which involves a combination of elemental zinc with the leaching solution containing the gold-cyanide complex), extraneous copper materials in the solution will dramatically reduce the precipitation efficiency of the system by causing zinc passivation, with the term "passivation" involving a process in which the zinc is rendered non-reactive to the gold-cyanide complex which prevents the gold precipitation process from taking place. Additional zinc will therefore be required which again increases overall production costs. Excessive copper contamination of the leaching solution will also reduce the operating efficiency of the smelting process associated with this embodiment by causing prolonged smelting times. In systems which employ the Activated Carbon Process, copper materials (e.g. copper-cyanide complexes) will substantially inhibit the functional capabilities of the activated carbon, thereby "fouling" this material and causing increased carbon requirements. Current efficiency and consumption are likewise increased in subsequent electrowinning stages if copper materials are not removed from the system. Accordingly, the presence of copper-containing species in the leaching solution after the treatment of gold ore will cause a number of significant problems unless the copper is effectively removed.
The present invention involves a unique and specialized procedure for removing undesired copper (e.g. copper-cyanide complexes) from gold extraction systems when copper-containing gold ore is being processed. As a result, production costs are greatly reduced which contributes to a substantial increase in overall operating efficiency. The claimed process is readily applicable to a wide variety of cyanide-based treatment methods ranging from heap leaching to vat leaching. It may also be used with many different methods for isolating elemental gold from gold-cyanide complexes including the Merrill-Crowe Process, the Activated Carbon Process, and others. The claimed method is highly versatile, satisfies a long-felt need in the gold processing industry, and provides the following important benefits: (1) the ability to process impure, copper-containing gold ore in an economical manner without the excessive consumption of cyanide compositions (e.g. free cyanide (CN).sup.- !); (2) an improvement in the operating efficiency of the entire gold processing system by reducing cyanide reagent costs; (3) the decreased consumption of other reagents in the system including activated carbon and zinc (depending on the particular recovery system under consideration); (4) a reduction in electricity consumption (if electrowinning is part of the overall processing system); (5) improved conservation of resources and reduced waste generation which collectively provide important environmental benefits; (6) a reduction in the amount of smelting time that is needed to yield an elemental gold product; (7) the ability to retain, purify, and collect elemental copper from the gold ore which can be sold at considerable economic benefit; (8) a high level of versatility and applicability to a wide variety of different cyanide-based processing methods; (9) improved gold purity levels in connection with the gold product "dore"; and (10) a general improvement in the simplicity, effectiveness, and efficiency of the gold production system. For these reasons and the other factors outlined below, the present invention and its various embodiments represent a significant advance in the art of gold refining.